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Chapter 3 Elatroanalytical Methods Voltammetry

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Chapter 3 Elatroanalytical Methods Voltammetry Chapter 3 Elatroanalytical Methods Voltammetry 1 What is Voltammety? •A group of analytical methods in which only a small portion of material is electrolytically reduced or less commonly oxidized •Classification of voltammetric methods is based on: Electrode type How the potential is applied How the current is measured 2 Characteristics • Voltammetry is based upon the measurement of a current that develops in an electrochemical cell under conditions of complete concentration polarization. • Potentiometric measurements are made at currents that approach zero and where polarization is absent • Furthermore, in voltammetry a minimal consumption of analyte takes place, whereas in electrogravimetry and coulometry essentially all of the analyte is converted to another state • Voltammetry (particularly classical polarography) was an important tool used by chemists for the determination of inorganic ions and certain organic species in aqueous solutions. 3 Concept Detector/ Excitation signal voltage Sample Transducer/ Readout Process Sensor View Voltage is applied to Current is current as a analyte; appreciable transformed function of current is measured to voltage by time or electronics applied voltage Current is a function of • analyte concentration • how fast analyte moves to electrode surface • rate of electron transfer to sample • 4 voltage, time... II. Excitation process • A. What happens when a voltage is applied to an electrode in solution containing a redox species? generic redox species O • O + e- --> R E = -0.500 V v. SCE • Imagine that we have a Pt electrode in sol’n at an initial potential of 0.000 V v. SCE and we switch potential to -0.700 V. • First: O O supporting O E =0.0 Pt electrolyte app O 5 O O = redox solvent B. Events that happen 1. supporting electrolyte forms an electrical double layer O O O Eapp= -0.7 Pt O O double layer acts as a capacitor cation movement to electrode causes an initial spike in current Formation of double layer is good because it ensures that no electric field exists across whole sol’n (requires 100:1 conc ratio of supporting 6 elyte:redox species). 2. Electron transfer reaction O is converted to R at electrode surface. O O O R Eapp= -0.7 Pt O O O R { A depletion region of O develops - a region in which conc of O is zero. How does more O get to electrode surface? mass transport mechanisms 7 C. Mass transport to the electrode 1. Migration - movement in response to electric field. We add supporting electrolyte to make analyte’s migration nearly zero. (fraction of current carried by analyte zero) 2. Convection • stirring 3. Diffusion In experiments relying upon diffusion, no convection is desired, sol’n is quiescent. 8 Solutions and electrodes 1. Solutions: redox couple + solvent + supporting electrolyte • supporting elyte: salt that migrates and carries current, and doesn’t do redox in your potential window of interest • a wide potential window is desirable • water - good for oxidations, not reductions except on Hg supporting elytes: lots of salts • nonaqueous solvents: acetonitrile, dimethylformamide, etc. • supporting electrolytes: tetraalkylammonium BF4, PF6, ClO4 • Oxygen is fairly easily reduced - we remove it by deoxygenating with an inert gas (N2, Ar). 9 2. Electrodes • working etrode (WE) is where redox activity occurs • auxiliary etrode (AE) catches current flow from WE • reference etrode (RE) establishes potential of WE • a. working etrode materials: • Pt, Au, C, semiconductors • Hg - messy but good for reductions in water. Not good for oxidations. • b. auxiliary etrodes: similar materials, large in area • c. reference etrodes: real vs. quasi - • real refs have an actual redox couple (e.g. Ag/AgCl) • quasi refs (QRE) - a wire at which some (unknown) redox process occurs in sol’n. QREs OK if currents are needed but not potentials. 10 Voltammetric Techniques • Polarography • Square Wave Voltammetry • Cyclic Voltammetry • LSV • Differential Pulse • Normal Pulse • Sampled DC • Stripping Analysis 11 12 13 DC voltage source /ammeter ammeter Reference Cathode Working electrode indicator electrode 14 15 Supporting Electrolyte • Polaragrams are recorded in the presence of a relatively high concentration of a base electrolyte such as KCI. • The base electrolyte will decrease the resistance for the movement of the metal ions to be determined thus, the IR drop throughout the cell will be negligible. • It helps also the movement of ions towards the electrode surface by diffusion only. • The discharge potential of the base electrolyte takes place at a very low negative potential therefore, most ions will be reduced before the base electrolyte species. • Buffering & elimination of interferent by complexation 16 17 18 Static Mercury Drop Electrode 19 20 Model 394 Voltammetric Analyzer Computer • controlled polarographic and voltammetric analyzer PC compatible • Windows software Can use existing • 303A / 305 21 Wide Range of Techniques • Square Wave Voltammetry • Cyclic Voltammetry • LSV • Differential Pulse • Normal Pulse • Sampled DC • Stripping 22 Analysis Pre-experiment selection • Analyzer consol controls SMDE • Automatic purging and stirring of sample • Automatic conditioning of electrode • Automatic control of 23 deposition times Standards • Up to nine standards can be entered • Selection of common reference electrode potentials • Electrolyte 24 record Multi-element Analysis 25 26 27 28 29 30 31 Advantages and Disadvantages of the Dropping Mercury Electrode • High overvoltage associated with the reduction of hydrogen ions. As a consequence, metal ions such as zinc and cadmium can be deposited from acidic solution even though their thermodynamic potentials suggest that deposition of these metals without hydrogen formation is impossible. • A second advantage is that a new metal surface is generated continuously; thus, the behavior of the electrode is independent of its past history. In contrast, solid metal electrodes are notorious for their irregular behavior, which is related to adsorbed or deposited impurities. • A third unusual feature of the dropping electrode, which has already been described, is that reproducible average currents are immediately realized at any given potential whether this potential is approached from lower or higher settings. 32 • One serious limitation of the dropping electrode is the ease with which mercury is oxidized: this property severely limits the use of the electrode as an anode. At • potentials greater than about + 0.4 V, formation of mercury(I) gives a wave that masks the curves of other oxidizable species. • In the presence of ions that form precipitates or complexes with mercury(I), this behavior occurs at even lower potentials. For example, in the Figure, the beginning of an anodic wave can be seen at 0 V due to the reaction - - • 2Hg + 2CI < === > Hg2CI2(s) + 2e • Incidentally, this anodic wave can be used for the 33 determination of chloride ion. • Fig. 18: Residual current curve for a 0.1M solution of HCl 34 • Another important disadvantage of the dropping mercury electrode is the nonfaradaic residual or charging current, which limits the sensitivity of the classical method to concentrations of about 10- 5 M. • At lower concentrations, the residual current is likely to be greater than the diffusion current, a situation that prohibits accurate measurement of the latter. • Finally, the dropping mercury electrode is cumbersome to use and tends to malfunction as a result of clogging. 35 Effect of Dissolved Oxygen • Oxygen dissolved in the solution will be reduced at the DME leading to two well defined waves which were attributed to the following reactions: + - O2(g) + 2H + 2e < ==== > H2O2 + - H2O2 + 2H +2e < ==== > 2H2 O • E1/2 values for these reductions in acid solution correspond to -0.05V and -0.8V versus SCE . 36 37 38 39 40 41 42 Polarogram for (A) 5X10-4M Cd2+ in 1M HCl. (B) 1M HCl 43 44 708 1 / 2 2 / 3 1/6 Id (average) = 6/7 (708) n D m t C 45 46 47 48 49 50 51 Currents controlled by factors other than diffusion • Processes other than diffusion are involved on the electrode surface: 1. Chemical reactions involving oxidation or reduction 2. Adsorption of electroactive species 52 Kinetic Currents • Currents whose magnitudes are controlled by that rates of chemical reactions A (not electroactive) + X Ox k A + X 1 Ox + ne R CH2O(H2O) CH2O + H2O + CH2O + 2H + 2e CH3OH il = id + ik 53 Catalytic Current • It is controlled by a catalytic process • Either the electroactive substance is regenerated by a chemical reaction 3+ 2+ 3+ Fe + e Fe + H2O2 Fe • The electroreduction of a species is shifted to a more +ve potential • Proteins catalyze the reduction of H+ and shift the corresponding wave to a more +ve potential • ik is a nonlinear function of concentration or linear over a limited concentration range 54 Adsorption Currents • If oxidized form is adsorbed its reduction will take place at a more –ve potential than the diffusion current • If reduced form (product) is adsorbed its reduction will take place at a more +ve (prior) potential than the diffusion current 55 Polarographic Maxima • Currents that are at a certain point of potential higher (about 2 order of magnitude) than the diffusion current • Be removed by addition of surfactant (triton-100) or gelatin 56 Tests of Current Limiting Processes • Usually the currents are distinguished from each other by the changes that take place when the following parameters are varied: • Concentration of electroactive species • Mercury column height • pH • Buffer concentration •
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